Obesity - Causes and Consequences

Are you worried that you or someone close to you might be becoming too fat or obese? What health problems might this lead to? What are the symptoms?

What is Obesity? 

Obesity means being extremely overweight and carrying too much body fat ie with a Body Mass Index (BMI) of over 30. 

A BMI of between 20 and 25 is considered ideal. BMIs between 25 and 30 are considered overweight and BMIs between 30 and 39.9 obese. This is a useful rough guide, although the BMI calculation can’t distinguish between excessive fat or if someone is muscular.  

The Food Standards Agency has created a useful height/weight chart. Simply step on the scales and check how you compare. 

Is obesity potentially life-threatening? 

One estimate is that if you’re over 40 and obese you’ll die 6–7 years sooner than you would if you had been a normal weight. 

That’s partly because of the extra strain this puts on your heart. The more fatty tissue you’re carrying the more blood your heart has to pump, which can lead to it effectively wearing out. One major, long term study in the US, concluded that being obese doubled your risk of heart failure. 

NHS Wales advises that being obese can lead to a number of serious health problems as well as shortening your life expectancy. It explains that obesity increases a range of health risks, including: 

  • High blood pressure (hypertension) - a major risk factor for developing a serious cardiovascular disease
  • Coronary heart disease – the UK’s biggest killer (causing around 94,000 deaths a year)
  • Type 2 diabetes - a chronic (long-term) condition
  • Cancer - including breast cancer (the most common cancer in the UK) and bowel cancer (another common cancer)
  • Stroke – when blood supply to the brain is interrupted and immediate treatment is needed to limit potentially fatal damage 

In fact Diabetes UK report that obesity is believed to account for 80 - 85% of the risk of developing type 2 Diabetes.

Other health risks include infertility, osteoarthritis, chronic back pain and depression. 

How is obesity caused? 

Eating too much while not getting enough exerciseis is often cited as the most common cause i.e. regularly consuming more calories than we burn off.

As we report elsewhere on the website, exercise has many health benefits.

However, research now suggests that it is how much food we eat and what type of food that is primarily causing obesity - and that this in turn reflects changes in the global food system, including food now being available at a much lower time cost. 

Food high in sugar and fat is a particular risk. For example sugar sweetened drinks add calories without making us feel full, so we tend to eat as much as usual without realising that the soft drinks have been adding extra 'hidden' calories.

Eating whenever food was available probably made evolutionary sense. For thousands of years people could never be sure where the next meal was coming from, so it made sense to eat whenever food was available. 

However, today food is readily available from many sources, from fast food outlets to supermarkets. That's one of the changes in the global food system we mentioned earlier. That evolutionary impulse to eat whenever food is available could now be killing us. 

Research also suggests that what type of food we eat also matters. People who are obese haven't usually been binge eating vegetables, fruit and wholegrains. They are more likely to have been eating convenience foods high in sugar, salt, fat and refined carbohydrates. For example one review of 50 studies estimated that increasing fibre intake by 14 grams oer day was associated with a 2 kilogram weight loss over about a 4 month period - and fibre is more common in vegetables, fruit and wholegrains.

Experiments in mice have suggested there may be a ‘fat gene’ which encourages some people to eat more. If this proves to be the case in humans it would suggest some people may face more of a challenge than others.However, large scale obesity is a recent trend historically and our genes don't change that quickly. This suggests that if there is a 'fat gene' it was only triggered relatively recently as food became so abundantly available.  


  • A BMI of 30 – 39.9 is considered obese.
  • Being obese could knock 6 – 7 years off your life – it increases your risk of heart attack, stroke, diabetes and cancer.
  • The main cause of obesity is eating too much of the wrong type of food.

To find out more, read our guide to Preventing Obesity

Delia Morick

Published 26/9/13. Updated 02/07/15. Review date May 2018

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